The report added that the launch pad, which could be ready as early as March, would allow for "a more robust rocket test program in the future involving larger space launch vehicles and road-mobile ballistic missiles able to attack targets in Northeast Asia and the United States."

Director of National Intelligence James Clapper warned the Senate Intelligence Committee on Wednesday that North Korea had gone through with its threat to continue its nuclear weapons program, posing “a serious threat to the United States and to the security environment in East Asia.”

The satellite images appeared to show that North Korea has also tested a rocket engine for a new road-mobile intercontinental missile called the KN-08, the report said. Rockets have been displayed at parades in the capital Pyongyang but analysts have speculated these were mock-ups.

Outwardly, however, North Korea is on something of a charm offensive, having hosted the press in New York, London and Beijing in the past week.

North Korea's ambassador to the U.K., Hyun Hak-bong, gave an interview to Sky News on Thursday in which he blamed the U.S. for its hostile policies. He said his country had "no option but to have the nuclear deterrent."

It comes ahead of the annual joint military exercises between the U.S. and South Korea. Last year more than 10,000 American personnel participated. North Korean leader Kim Jong Un responded with a series of internationally-condemned missile tests and suggested he was about to launch a nuclear attack.